The goal of this project is to de?ne the cellular role of TMCO1, a glaucoma-linked gene which encodes an integral membrane protein of unknown function. More than two million Americans are affected by glaucoma, a group of neurodegenerative diseases involving optic nerve damage resulting from the apoptotic death of retinal ganglion cells (RGC). Despite its prevalence, little is known about the development and progression of glaucoma on the molecular level. Multiple genome-wide association studies in different populations have recently identi?ed associations between primary open angle glaucoma (POAG)-the most common form of glaucoma-and single nucleotide polymorphisms (SNPs) around a gene called TMCO1. Intriguingly, some of these SNPs are correlated with changes in expression levels of the TMCO1 gene. That such changes contribute to the pathogenesis of glaucoma is consistent with previous studies suggesting that TMCO1 is an apoptosis regulator expressed in multiple eye tissues, including RGCs. However, the role of TMCO1 in glaucoma remains unknown, because its cellular and molecular functions are completely unde?ned. We recently uncovered a possible function for TMCO1 as a result of a phylogenetic analysis. We found that TMCO1 belongs to a previously unrecognized superfamily of distantly related proteins that are broadly involved in membrane protein biogenesis. Furthermore, we have obtained functional data showing that TMCO1 localizes to the endoplasmic reticulum (ER) membrane where it interacts with ribosomes and other components of the known biosynthetic machinery. Based on these preliminary data, we hypothesize that TMCO1 functions to insert certain membrane proteins into the ER during their biosynthesis. An attractive model is that defects in TMCO1-dependent insertion lead to misfolding, accumulation of toxic aggregates and induction of apoptosis, hallmarks of many neurodegenerative diseases. Alternatively, the disease phenotype might be linked to failed biogenesis of a speci?c protein(s) that is essential for normal RGC survival. We are uniquely positioned to de?ne the cellular role of TMCO1 using quantitative proteomics, cell-based localization studies and in vitro biochemistry. In Aim 1 we will identify human membrane proteins whose biogenesis is dependent on TMCO1. In Aim 2 we will develop an in vitro insertion assay and demonstrate that TMCO1 is a bona ?de insertase. This is a high-risk project that promises high-impact payoff. By de?ning the function of TMCO1 and identifying substrates on which it acts, these studies will provide a molecular framework to understand how changes in TMCO1 expression are linked to the development of glaucoma. More broadly, if our hypothesis is correct, we will have discovered a new membrane protein insertion system in the endoplasmic reticulum. Such work would impact our understanding of membrane protein biogenesis and its role in human disease.